The Multiverse (aka the
Tellurian) is a concept featured in many works of Fantasy, Science Fiction and (weird but true) actual
real world science. To put it simply the Multiverse idea is that there are lots of parallel universes. In a book or game exploiting this idea it is usually possible to travel between them. In fantasy works (such as
Michael Moorcock's
Elric or
Corum books or the
Planescape RPG) these universes are usually called
Planes.
Notable books that include the Multiverse concept
as a central theme include:
The Coming of the Quantum Cat, Frederik Pohl (SF)
The Gods Themselves, Isaac Asimov (SF)
Nomad of the Timestreams, Michael Moorcock (SF)
Number of the Beast, Robert Heinlein (SF)
His Dark Materials, Phillip Pullman (often availble in three volumes: 1- Northern Lights aka The Golden Compass (US), 2- The Subtle Knife, 3- The Amber Spyglass) (Fantasy/SF)
Amber, Roger Zelazny (Fantasy/SF)
The Death Gate Cycle, Margaret Weis and Tracy Hickman (Fantasy)
The Blood Wars Trilogy, J. Robert King (Fantasy)
The Elric Saga, Michael Moorcock (Fantasy)
The Chronicles of Corum, Michael Moorcock (Fantasy)
The Hawkmoon Saga, Michael Moorcock (Fantasy)
The Lord Soth Cycle, James Lowder (Gothic Horror/Fantasy)
The Hitchhikers Guide to the Galaxy, Douglas Adams (Comedy/SF)
Lords and Ladies, Terry Pratchett (Describes the trousers of time theory) (Comedy/Fantasy)
Witches Abroad, Terry Pratchett (Comedy/Fantasy)
Soul Music, Terry Pratchett (Comedy/Fantasy)
Moving Pictures, Terry Pratchett (Comedy/Fantasy)
Plus pretty much everything else by Michael Moorcock (notably the Eternal Champion series) also strongly features the Multiverse.
Based on the Planescape idea Planes can be split into a number of major groups:
Prime planes are the planes of the standard book/game worlds such as the Marvel and D&D universes. There are an infinite number of Primes which include every 'what if' scenario you can imagine.
Voids are planes that are generally used to get from one place to another. They link the Primes with each other and with the Elemental and Outer Planes. In Planescape they are called the Ethereal Plane and the Astral Plane. In an SF setting such as Babylon 5 hyperspace would probably come under this heading.
The Elemental Planes are Planes that are each based around one of the basic elements; Earth, Air, Fire and Water. In addition there are the Positive Energy and Negative Energy Planes which are also considered part of the Elemental group.
The Outer Planes are planes that are based around a certain philosphy, notably a vision of heaven or hell. So Dante's Hell, The Happy Hunting Grounds and the Greek Hades would all come under this category. In Planescape the Outer Planes form a Great Wheel. Each Plane is based around an Alignment:- a balance between Good, Evil, Law and Chaos. In the hub of the Wheel is the plane of Concordant Opposition; Purgatory the Plane of perfect balance where all four forces are kept in exactly even measure. In the Planescape series the very centre of this plane (the Outlands) rises a huge spire above which floats Sigil a city which serves as a crossroads for the Multiverse.